Warm up the steak and spinach in a saute pan or in the microwave. Place all ingredients, except butter, leftovers and cheese in a blender and process on high speed for about 10 seconds to incorporate air into the mix; the club soda and baking powder will help the omelet to rise. (You may also use a hand-held blender with a deep cup).
Melt the butter in a large non-stick pan over medium-high heat. Pour in the egg mixture, and using a rubber spatula, slowly push the cooked egg from 1 side of the bottom of the pan to the other to allow new raw egg to reach the bottom of the pan and cook (this creates height and keeps the bottom from burning).
When the omelet is cooked on the bottom and the top is still runny, you can either flip the omelet in the pan to finish, or to make the "Tallest" omelet possible, place the pan just below the broiler with the oven door open and the handle sticking out (it could melt otherwise) for about 30 seconds until the omelet rises and lightly browns.
Remove from heat, add warmed leftover filling and the cheese, and fold in half. To serve, slice into pieces. If desired, save a little filling and cheese to put on top after folding.